TNPO3 is a cellular nuclear import receptor required for HIV-1 infection. Depletion of TNPO3 expression dramatically decreases the infection of primate lentiviruses such as HIV-1, HIV-2 and SIVmac. TNPO3 is essential for early steps on HIV-1 replication. The viral determinant for the requirement of TNPO3 in HIV-1 infection has been genetically mapped to the viral capsid. In agreement with these observations, we have demonstrated that TNPO3 specifically binds the HIV-1 core that is composed of capsid. These results imply that TNPO3 might be interacting with the incoming HIV-1 core early on infection. Depletion of TNPO3 moderately but consistently affects the uncoating process of HIV-1. It is believed that TNPO3 is involved in the nuclear import of the HIV-1 pre-integration complex (PIC); however, this interpretation is in question, as more recent evidence implies that TNPO3 assists HIV-1 replication after nuclear import. Considering the following evidence: 1) depletion of TNPO3 expression affects HIV-1 replication, 2) the viral determinant for the requirement of TNPO3 maps to capsid, 3) TNPO3 binds the HIV-1 capsid, 4) depletion of TNPO3 expression moderately but consistently affects uncoating, and 5) our recent observation that TNPO3 assists HIV-1 replication in an step after nuclear import but before integration. We will test the hypothesis that the binding of TNPO3 to the HIV-1 core in the cytoplasm allows the occurrence of a process that is required for productive infection when the pre-integration complex reaches the nucleus. This proposal will explore the contribution of TNPO3 to HIV-1 integration into the genome by monitoring the two pivotal processes mediated by HIV-1 integrase in the absence of TNPO3, namely 3'-prime processing and DNA strand transfer. Secondly, we will study the role of TNPO3 nuclear-import in HIV-1 infection. The goal of these studies are to understand the mechanism used by TNPO3 to assist HIV-1 replication with the future objective of using TNPO3 as a therapeutic target to block HIV-1 replication. !